Last year brought glum news that Canada's forests are only a so-so defence against global warming. Today, it gets a little worse: We thought our forests were getting better at soaking up greenhouse gases, but they're not.

A study by Canadian, Chinese and European researchers shows that as the climate gets warmer, northern forests aren't soaking up extra carbon dioxide from the air after all. Forests may, in fact, become worse at storing carbon if climate trends continue.

Canada has always argued that our forests strongly "offset" some of the fossil fuels we burn. Our official position is that Kyoto-style climate plans should give Canada credit for the good work our forests do.

However, a series of studies over the past two years, and continuing today in the journal Nature, is calling that into question.

Forests soak up less pollution that we'd hoped. But even as Canada realized it over-hyped the air-cleaning work done by forests, one apparent piece of good news emerged.

Scientists noticed that the global warming trend was waking up trees earlier each spring. As well, the trees were staying green longer into the autumn.

This longer growing season, they reasoned, meant that trees should work longer each year at building new branches and leaves - the process that soaks up carbon from the air.

So, shouldn't that get rid of more carbon dioxide? No, says today's study by the Global Carbon Project, a multinational science network that includes Canada.

The study looks at years of data-gathering - largely from Canadian forests - that record precisely how much carbon dioxide is in the air of a forest day by day.

In the past 20 years, two things happened. The autumn in many forests of Canada, Europe and China has warmed by about 1.1 degrees. And autumn forests are releasing carbon dioxide back into the air faster than they soak it up.

This trend is so strong, atmospheric scientist John Miller of the University of Colorado writes in Nature, that it "seems to largely cancel" the gains made through the earlier arrival of spring and its extra forest growth.